Three-dimensional (“3D”) printing of physical models is useful in a wide variety of settings. Some potential uses include production of anatomical bodies like bones for research and clinical applications, medical product development, machine design, and equipment design, to name just a few. 3D printing or rapid prototyping refers to a collection of technologies for producing physical parts directly from digital descriptions. Digital descriptions include output of any software that produces a 3D digital model. One example of such software is Computer-Aided Design (CAD) software. Creating a 3D digital model from a 3D image data set requires specialized imaging or CAD software. Rapid prototyping machines have been commercially available since the early 1990's, the most popular versions of which build a desired structure by adding building material layer-by-layer based on a digital three-dimensional model of the structure.
However, because of the amount of user interaction time involved and the complexity of data conversion process between image data formats and data formats supported by 3D printers or rapid prototyping machines, applications of the present technology of producing 3D physical models from three or n-dimensional images are rather limited.
FIG. 1. illustrates the current method of creating a physical model from an input image data set. The input image data set comes in the form of 3D voxel data or serial, sequenced two-dimensional (“2D”) images. A voxel (volumetric pixel or, more correctly, Volumetric Picture Element) is a volume element on a regular grid in a three dimensional space, having one or more numerical values as attributes such as intensity or color. This is analogous to a pixel (Picture Element), which has one or more numerical values as attributes on a regular grid in a 2D image data set. A 3D image data set may be organized as a series of 2D images and a voxel in a two-dimensional image plane may be referred to as a pixel.
In FIG. 1, when a user 11 needs to create a physical model 35 from an image data set 10, the user 11 looks up the image on his/her computer 15 and transfers the image data 10 to an image processing operator 21. The image processing operator 21 loads the image data 10 set on his computer 20 where special image modeling software is available. The image processing operator 21 reads the instructions sent by the user 11 to understand what type of model is required. If the image processing operator 21 still has questions or needs additional information, he will communicate with the user 11 to get the information. The image processing operator 21 then starts the process to create a 3D digital model 22 from the image data sets 10 on his computer 20 using specialized modeling software. The creation of the 3D digital model 22 requires a trained operator 21, specialized imaging software, and a significant amount of user interaction. The image processing operator 21 needs to communicate frequently with the user 11 who has ordered the physical model to understand the requirements and applications of the model. The image processing operator is also required to spend a significant amount of time to perform image segmentation and to trace manually certain image areas. After the 3D digital model 22 has been created, it is then saved to a file format supported by a 3D printer or rapid prototyping machine 30, for example, the STL (stereolithography) file format. The digital model file is then sent to the 3D printer or rapid prototyping machine 30 to generate a 3D physical model 35. The three-dimensional (“3D”) printer 30 is likely located at a different location and operated by a 3D printing operator 31. When the physical model 35 is printed or fabricated, the 3D printing operator 31 sends it to the imaging processing operator 21 who then sends the finished physical model 35 back to the original user 11. The present 3D printing techniques are complex and cost ineffective. The physical models may take too long to create to be useful, for example, to an emergency-care doctor.
As a particular example of the need for an efficient 3D physical model printing process, we consider 3D printing applications in the medical field. In a typical application of 3D printing techniques in the medical field, medical images are first ordered and acquired on a hospital computer by a doctor. The doctor then sends the images to a trained image processing operator to create a digital model. The image processing operator communicates with the doctor to understand the requirements for the model. The image processing operator loads the image data set into a 3D image processing software to identify features such as bones, tissues, etc. by using image segmentation software tools. Because image processing of medical data is complex and time-consuming, it remains a challenging task even to a professional image processing operator.
After loading the image data, the image processing operator 21 then creates a digital 3D model, for example, a 3D polygonal surface model by using software-based modeling tools. As an example, one commercially available software solution, “3D-DOCTOR”, can be used to produce 3D digital models of anatomical structures, as described in Yecheng Wu, From CT Image to 3D Model, Advanced Imaging, Aug. 2001, 20-23. After creating the digital 3D model, the image processing operator 21 sends the digital model to a 3D printing service provider. The 3D printing operator 30 at the 3D printing service provider loads the digital model data on his computer, controls the 3D printer to produce a physical model, and then delivers the finished physical model to the doctor who ordered the model. The above-described process is user intensitive and requires operators to possess advanced software training, knowledge of the intended applications, and a good understanding of the difference between image data formats and the various data formats supported by 3D printer and rapid prototyping machines.
In the above described process, one procedure employed in image processing is image segmentation. Image segmentation refers to the delineation and labeling of specific image regions in an image data set that defines distinct structures. Image segmentation may include steps such as differentiating a particular structure from adjacent material having different composition and identifying distinct objects having the same or similar composition. For example, when constructing bone models from Computerized Tomography (“CT”) and/or Magnetic Resonance (“MR”) images, bony structures need to be delineated from other structures (soft tissues, blood vessels, etc.) in the images. Also, each bone must typically be separated from adjacent bones when modeling anatomical structures such as cervical spine or foot.
In 3D printing applications in the medical field, a useful feature is the capability of building a prototype of a patient-specific anatomical region quickly. For example, if a patient comes in with a broken ankle, the surgeon may use a physical model of the bone fragments of the patient to aid surgical planning, if the physical model can be generated rapidly. For orthopedic surgeons, the ability to visualize and manipulate a physical model of a bone or joint in need of repair prior to surgery can aid in the selection and design of surgical implants for fracture fixation or joint replacement. Rapid prototyping of patient specific models increases efficiency and reduces costs by cutting operating room time. Rapid prototyping of patient specific models offers tremendous promise for improved pre-operative planning and preparation. While the technique of sizing surgical implants using newer imaging modalities such as Computerized Tomography (“CT”) and/or Magnetic Resonance (“MR”) imaging is an improvement over standard X-ray films, the ability to work with an accurate physical model of the region of interest would produce further benefits, such as providing tactile 3D feedback of the relevant patient anatomy. Rapid prototyping or 3D printing refers to a collection of technologies for producing physical parts directly from digital descriptions, which frequently are the output from Computer-Aided Design (CAD) software. Rapid prototyping machines have been commercially available since the early 1990's, and the most popular versions involve adding material to build the desired structure layer-by-layer based on a digital three dimensional model of the structure. For example, a physical model may be fabricated using a rapid prototyping system using stereolithography, fused deposition modeling, or three dimensional printing. In stereolithography, a laser is used to selectively cure successive surface layers in a vat of photopolymer. In fused deposition modeling, a thermal extrusion head is used to print molten material (typically a thermoplastic) that fuses onto the preceding layer. A typical three-dimensional printer uses a printer head to selectively deposit binder onto the top layer of a powder bed.